High efficiency single inductor buck-boost DC-DC converters are well known in the art. An example of these types of DC-DC converters are the integrated semiconductor electronic devices of the TPS 630xx series of Texas Instruments, which provide a power supply solution for products powered by either a two-cell or three-cell alkaline, NiCd or NiMH battery, or a one-cell Li-ion or Li-polymer battery. Output currents can go as high as 3 A, while using a single-cell Li-ion or Li-polymer battery and discharge it down to 2.5 V or lower. This known type of buck-boost converters is based on a fixed frequency, pulse-width-modulation (PWM) controller using synchronous rectification to obtain maximum efficiency. At low load currents, the converter enters power save mode to maintain high efficiency over a wide load current range. The power save mode can be disabled, forcing the converter to operate a fixed switching frequency. The maximum average current in the switches is limited to a typical value of 4 A. The output voltage is programmable using an external resistor divider, or it is fixed internally on the chip.
In order to optimize efficiency and performance of certain applications of DC-DC conversion; for example, for power amplifiers in cellular phones, it is required to change the supply voltage level in accordance with the required output power.
Typical DC-DC converters, as the previously described DC-DC converters using the TPS630xx, series generate a precise output voltage from an input voltage that can vary within a specified range. The value of the output voltage is defined by a reference voltage. In order to change the output voltage, either the feedback divider or the reference voltage has to be changed to a new value. In both cases, the response at the output node of the DC-DC converter is limited by the bandwidth of the control loop of the DC-DC converter.